function   RR=checkchildren(j,RR)
% if a symbol 't' is encounted, then make all its descendants in reference  
% matrix RR's components equal 1---ZEROTREES
global N
[m,n]=size(N);
for i=(4*j-3):4*j;
      if  i<=m,      
         RR(N(i,1),N(i,2))=1;
         RR=checkchildren(i,RR);
      end
end;

   



